Silicon is obtained mainly by carbothermal reduction of silicon dioxide in an arc furnace. The product obtained, so-called metallurgical silicon, contains 96 to 99% by weight of silicon and various impurities consisting mainly of the elements aluminium, iron, boron, phosphorus and carbon and compounds of these elements.
Large quantities of inexpensive silicon are required for use in photovoltaic processes.
This silicon, so-called silicon of solar quality, must satisfy certain standards of purity (see J. Dietl, D. Helmreich, and E. Sirtl; Solar Silicon, in J. Grabmaier (publishers): Crystals, Growth, Properties, Applications, Vol. 5, page 45, Berlin-Heidelberg-New York, Springer 1981).
No process has so far become known by which metallurgical silicon can be purified to the required degree in a single step. It has always required a multistage process of purification.
The metal impurities may be removed by an acid treatment according to U.S. Pat. No. 2,402,839, and the phosphorus content can be reduced to the required concentrations by a vacuum treatment. However, the carbon content is difficult to reduce to the required low concentrations and the necessary reduction of the boron content is virtually impossible.
Metallurgical silicon having a degree of purity of about 98% generally contains 700 to 1500 ppma of carbon (ppma =atoms of carbon per 1 million atoms of silicon).
This concentration is far above the tolerance limit permissible for use in photovoltaics. It is therefore absolutely necessary to reduce the carbon content.
Published European Application 137,306 reports on the removal of carbon from solid silicon fragments by treating the fragments with aqueous nitrate solution and annealing at 1000.degree. to 1100.degree. C.
This process, however, is unable to reduce the carbon content sufficiently.
German Pat. No. 3,403,131 describes a process for the removal of unreduced quartz and unreacted carbon from silicon produced in an arc furnace by melting the silicon in a graphite or quartz crucible. The unreduced quartz and unreacted carbon collect on the wall of the crucible and remain there when the silicon is poured off. Even this process, however, cannot provide the necessary degree of purity.
German Pat. No. 3,416,559 describes a process in which the silicon carbide and silicon dioxide particles present in molten silicon are mechanically removed by centrifuging, but this process is very expensive.
It is an object of the present invention to provide a process for the preparation of low carbon silicon from molten silicon which is free from the disadvantages described above.